Memory System Configured to Avoid Memory Access Hazards for LDPC Decoding

ABSTRACT

Techniques are disclosed relating to resolving memory access hazards. In one embodiment, an apparatus includes a memory and circuitry coupled to or comprised in the memory. In this embodiment, the circuitry is configured to receive a sequence of memory access requests for the memory, where the sequence of memory access requests is configured to access locations associated with entries in a matrix. In this embodiment, the circuitry is configured with memory access constraints for the sequence of memory access requests. In this embodiment, the circuitry is configured to grant the sequence of memory access requests subject to the memory access constraints, thereby avoiding memory access hazards for a sequence of memory accesses corresponding to the sequence of memory access requests.

PRIORITY DATA

This application claims the benefit of U.S. Provisional Application No.62/045,832, titled “Memory Configured to Avoid Access Hazards,” filed onSep. 4, 2014, whose inventors are Tai A. Ly, Swapnil D. Mhaske, HojinKee, Adam T. Arnesen, David C. Uliana, and Newton G. Petersen, which isincorporated by reference herein in its entirety as though fully andcompletely set forth herein.

TECHNICAL FIELD

The present disclosure relates to the field of computer memory, and moreparticularly to techniques involving a memory system configured toresolve or prevent memory access hazards.

DESCRIPTION OF THE RELATED ART

Various computer programs require handling of hazards when accessingmemory. Generally, when two operations access the same memory location,a computing system should ensure that the accesses occur in the properorder. Various algorithms use sequences of memory operations that areoften performed iteratively. For example, error control codes,encryption, and various equation solving algorithms typically usecomplex matrices for which memory accesses must be performed inspecified sequences for various operations.

Low-density parity-check (LDPC) codes are one example of error controlcode and are used to transmit information over potentially noisychannels. For example, the WiMAX and LTE standards utilize LDPC codesfor data channels. LDPC typically uses a parity check matrix referred toas an “H matrix” to determine the makeup of incoming blocks of code.Decoding LDPC encoded messages involves iterative read/modify/writeoperations corresponding to different layers of the H matrix. Pipeliningthese operations may be desirable in order to decrease decode time, butdependencies between the memory accesses must be resolved to properlyorder the pipelined operations.

SUMMARY

Various embodiments of a memory configured to resolve or prevent accesshazards, and techniques for configuring or using such a memory, arepresented below.

In one embodiment, an apparatus includes a memory and circuitry coupledto or comprised in the memory. In this embodiment, the circuitry isconfigured to receive a sequence of memory access requests for thememory, where the sequence of memory access requests is configured toaccess locations associated with entries in a matrix. In thisembodiment, the circuitry is configured with memory access constraintsfor the sequence of memory access requests. In this embodiment, thecircuitry is configured to grant the sequence of memory access requestssubject to the memory access constraints, thereby avoiding memory accesshazards for a sequence of memory accesses corresponding to the sequenceof memory access requests.

In some embodiments, the apparatus includes pipeline circuitryconfigured to pipeline performance of the sequence of memory accesses.In these embodiments, the circuitry may be configured to stall pipelinedperformance of the sequence of memory access requests based on thememory access constraints. The memory access constraints may bespecified using any of various formats and encodings. For example, insome embodiments interlock information indicates read/write pointervalues. In other embodiments, interlock information indicates tokenvalues. In some embodiments, the sequence of memory access requests isbased on iterative operations for processing rows of a matrix. In someembodiments, the circuitry is configured to prevent memory accesshazards without receiving other information indicating the memory accesshazards, i.e., without auxiliary information describing orcharacterizing the memory access hazards.

In one embodiment, a method includes determining a sequence of memoryaccess requests for a program and generating information specifyingmemory access constraints based on the sequence of memory accesses,where the information is usable to avoid memory access hazards for thesequence of memory accesses. In this embodiment, the method furtherincludes configuring first circuitry using the information, where thefirst circuitry is included in or coupled to a memory. In thisembodiment, after the configuring, the first circuitry is operable toperform memory access requests to the memory corresponding to thesequence of memory accesses while avoiding the memory access hazards,without receiving other information indicating the memory accesshazards. The configuring the first circuitry may be performed using aprogrammable hardware element, such as a field programmable gate array(FPGA), or other programmable hardware. Furthermore, the configuring maybe performed at compilation time, at start of running the program, or atvarious times while the program is running to adapt to changingconditions or to improve any number of performance measures such asthroughput, latency, power consumption, reliability, resourceutilization, bit-error rate, etc. In some embodiments, the determiningis performed using compile-time execution.

In one embodiment, a method includes storing a specified sequence ofmemory accesses that corresponds to a function to be performed. In thisembodiment, the specified sequence of memory accesses has first memoryaccess constraints. In this embodiment, the method further includesreordering the specified sequence of memory accesses to create areordered sequence of memory accesses that has second, different memoryaccess constraints. In this embodiment, the reordered sequence of memoryaccesses is usable to access a memory to perform the function. In someembodiments, performance estimates are determined for a plurality ofreordered sequences of memory accesses, and one of the reorderedsequences is selected based on the performance estimates. In someembodiments, the reordered sequence is used to compile a program usableto perform the function.

In one embodiment, an apparatus includes a memory and addressingcircuitry coupled to or comprised in the memory. In this embodiment, theaddressing circuitry is configured to receive memory access requestscorresponding to a specified sequence of memory accesses. In thisembodiment, the memory access requests do not include addressinformation. In this embodiment, the addressing circuitry is furtherconfigured to assign addresses to the memory access requests for thespecified sequence of memory accesses. In some embodiments, theapparatus is configured to perform the memory access requests using theassigned addresses.

Various techniques disclosed herein may improve memory accessthroughput, reduce complexity in devices coupled to an interlock memory,and/or allow for flexible programming or updating of an interlock memoryfor different applications, in some embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present disclosure can be obtained whenthe following detailed description is considered in conjunction with thefollowing drawings, in which:

FIG. 1A illustrates a computer system configured to execute a programaccording to an embodiment of the present disclosure;

FIG. 1B illustrates a network system comprising two or more computersystems that may implement an embodiment of the present disclosure;

FIG. 2A illustrates an instrumentation control system according to oneembodiment of the disclosure;

FIG. 2B illustrates an industrial automation system according to oneembodiment of the disclosure;

FIG. 3A is a high level block diagram of an exemplary system which mayexecute or utilize programs including graphical programs;

FIG. 3B illustrates an exemplary system which may perform control and/orsimulation functions;

FIG. 4 is an exemplary block diagram of the computer systems of FIGS.1A, 1B, 2A and 2B and 3B;

FIG. 5 illustrates one embodiment of a system that includes a decoderwith an interlock memory;

FIG. 6A illustrates an exemplary H matrix;

FIG. 6B illustrates one embodiment of a beta matrix and FIG. 6Cillustrates one embodiment of a rearranged beta matrix;

FIGS. 7A and 7B illustrate exemplary baseline decoding and pipelineddecoding timelines respectively;

FIGS. 8A and 8B illustrate respective embodiments of an interlockmemory;

FIG. 9 illustrates one embodiment of an interlock table;

FIG. 10 illustrates one embodiment of a method for operating aninterlock memory;

FIG. 11 illustrates one embodiment of a method for generatinginformation for an interlock memory using compile-time-execution;

FIG. 12 illustrates one embodiment of a method for determiningperformance estimates and rearranging memory access sequences based onperformance estimates;

FIG. 13 illustrates one embodiment of a method for configuring aninterlock memory;

FIG. 14 is a block diagram illustrating another embodiment of aninterlock memory;

FIGS. 15A and 15B illustrate exemplary beta matrices, interlock tables,and pipelined execution of memory access operations according to oneembodiment;

FIGS. 16A and 16B illustrate another embodiment of a method foroperating an interlock memory;

FIG. 17 illustrates one embodiment of a method performed using aninterlock memory;

FIG. 18 illustrates one embodiment of a method for reordering sequencesof memory accesses;

FIG. 19 illustrates one embodiment of a method performed using aself-addressing memory; and

FIG. 20 illustrates a sequence of memory accesses for Choleskyfactorization of a symmetric matrix.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and are herein described in detail. It should beunderstood, however, that the drawings and detailed description theretoare not intended to limit the invention to the particular formdisclosed, but on the contrary, the intention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present disclosure as defined by the appended claims.

The term “configured to” is used herein to connote structure byindicating that the units/circuits/components include structure (e.g.,circuitry) that performs the task or tasks during operation. As such,the unit/circuit/component can be said to be configured to perform thetask even when the specified unit/circuit/component is not currentlyoperational (e.g., is not on). The units/circuits/components used withthe “configured to” language include hardware—for example, circuits,memory storing program instructions executable to implement theoperation, etc. Reciting that a unit/circuit/component is “configuredto” perform one or more tasks is expressly intended not to invoke 35U.S.C. §112(f) for that unit/circuit/component.

DETAILED DESCRIPTION Incorporation by Reference

The following references are hereby incorporated by reference in theirentirety as though fully and completely set forth herein:

-   U.S. Pat. No. 4,914,568 titled “Graphical System for Modeling a    Process and Associated Method,” issued on Apr. 3, 1990.-   U.S. Pat. No. 5,481,741 titled “Method and Apparatus for Providing    Attribute Nodes in a Graphical Data Flow Environment”.-   U.S. Pat. No. 6,173,438 titled “Embedded Graphical Programming    System” filed Aug. 18, 1997.-   U.S. Pat. No. 6,219,628 titled “System and Method for Configuring an    Instrument to Perform Measurement Functions Utilizing Conversion of    Graphical Programs into Hardware Implementations,” filed Aug. 18,    1997.-   U.S. Pat. No. 7,210,117 titled “System and Method for    Programmatically Generating a Graphical Program in Response to    Program Information,” filed Dec. 20, 2000.-   U.S. patent application Ser. No. 13/592,995 titled “Compile Time    Execution,” filed Aug. 23, 2012.

Terms

The following is a glossary of terms used in the present application:

Memory Medium—Any of various types of non-transitory computer accessiblememory devices or storage devices. The term “memory medium” is intendedto include an installation medium, e.g., a CD-ROM, floppy disks 104, ortape device; a computer system memory or random access memory such asDRAM, DDR RAM, SRAM, EDO RAM, Rambus RAM, etc.; a non-volatile memorysuch as a Flash, magnetic media, e.g., a hard drive, or optical storage;registers, or other similar types of memory elements, etc. The memorymedium may comprise other types of non-transitory memory as well orcombinations thereof. In addition, the memory medium may be located in afirst computer in which the programs are executed, or may be located ina second different computer which connects to the first computer over anetwork, such as the Internet. In the latter instance, the secondcomputer may provide program instructions to the first computer forexecution. The term “memory medium” may include two or more memorymediums which may reside in different locations, e.g., in differentcomputers that are connected over a network.

Carrier Medium—a memory medium as described above, as well as a physicaltransmission medium, such as a bus, network, and/or other physicaltransmission medium that conveys signals such as electrical,electromagnetic, or digital signals.

Programmable Hardware Element—includes various hardware devicescomprising multiple programmable function blocks connected via aprogrammable interconnect. Examples include FPGAs (Field ProgrammableGate Arrays), PLDs (Programmable Logic Devices), FPOAs (FieldProgrammable Object Arrays), and CPLDs (Complex PLDs). The programmablefunction blocks may range from fine grained (combinatorial logic or lookup tables) to coarse grained (arithmetic logic units or processorcores). A programmable hardware element may also be referred to as“reconfigurable logic”.

Software Program—the term “software program” is intended to have thefull breadth of its ordinary meaning, and includes any type of programinstructions, code, script and/or data, or combinations thereof, thatmay be stored in a memory medium and executed by a processor. Exemplarysoftware programs include programs written in text-based programminglanguages, such as C, C++, PASCAL, FORTRAN, COBOL, JAVA, assemblylanguage, etc.; graphical programs (programs written in graphicalprogramming languages); assembly language programs; programs that havebeen compiled to machine language; scripts; and other types ofexecutable software. A software program may comprise two or moresoftware programs that interoperate in some manner. Note that variousembodiments described herein may be implemented by a computer orsoftware program. A software program may be stored as programinstructions on a memory medium.

Hardware Configuration Program—a program, e.g., a netlist or bit file,that can be used to program or configure a programmable hardwareelement.

Program—the term “program” is intended to have the full breadth of itsordinary meaning. The term “program” includes 1) a software programwhich may be stored in a memory and is executable by a processor or 2) ahardware configuration program useable for configuring a programmablehardware element.

Graphical Program—A program comprising a plurality of interconnectednodes or icons, wherein the plurality of interconnected nodes or iconsvisually indicate functionality of the program. The interconnected nodesor icons are graphical source code for the program. Graphical functionnodes may also be referred to as blocks.

The following provides examples of various aspects of graphicalprograms. The following examples and discussion are not intended tolimit the above definition of graphical program, but rather provideexamples of what the term “graphical program” encompasses:

The nodes in a graphical program may be connected in one or more of adata flow, control flow, and/or execution flow format. The nodes mayalso be connected in a “signal flow” format, which is a subset of dataflow.

Exemplary graphical program development environments which may be usedto create graphical programs include LabVIEW®, DasyLab™, DIADem™ andMatrixx/SystemBuild™ from National Instruments, Simulink® from theMathWorks, VEE™ from Agilent, WiT™ from Coreco, Vision Program Manager™from PPT Vision, SoftWIRE™ from Measurement Computing, Sanscript™ fromNorthwoods Software, Khoros™ from Khoral Research, SnapMaster™ from HEMData, VisSim™ from Visual Solutions, ObjectBench™ by SES (Scientific andEngineering Software), and VisiDAQ™ from Advantech, among others.

The term “graphical program” includes models or block diagrams createdin graphical modeling environments, wherein the model or block diagramcomprises interconnected blocks (i.e., nodes) or icons that visuallyindicate operation of the model or block diagram; exemplary graphicalmodeling environments include Simulink®, SystemBuild™, VisSim™,Hypersignal Block Diagram™, etc.

A graphical program may be represented in the memory of the computersystem as data structures and/or program instructions. The graphicalprogram, e.g., these data structures and/or program instructions, may becompiled or interpreted to produce machine language that accomplishesthe desired method or process as shown in the graphical program.

Input data to a graphical program may be received from any of varioussources, such as from a device, unit under test, a process beingmeasured or controlled, another computer program, a database, or from afile. Also, a user may input data to a graphical program or virtualinstrument using a graphical user interface, e.g., a front panel.

A graphical program may optionally have a GUI associated with thegraphical program. In this case, the plurality of interconnected blocksor nodes are often referred to as the block diagram portion of thegraphical program.

Node—In the context of a graphical program, an element that may beincluded in a graphical program. The graphical program nodes (or simplynodes) in a graphical program may also be referred to as blocks. A nodemay have an associated icon that represents the node in the graphicalprogram, as well as underlying code and/or data that implementsfunctionality of the node. Exemplary nodes (or blocks) include functionnodes, sub-program nodes, terminal nodes, structure nodes, etc. Nodesmay be connected together in a graphical program by connection icons orwires.

Data Flow Program—A Software Program in which the program architectureis that of a directed graph specifying the flow of data through theprogram, and thus functions execute whenever the necessary input dataare available. Said another way, data flow programs execute according toa data flow model of computation under which program functions arescheduled for execution in response to their necessary input databecoming available. Data flow programs can be contrasted with proceduralprograms, which specify an execution flow of computations to beperformed. As used herein “data flow” or “data flow programs” refer to“dynamically-scheduled data flow” and/or “statically-defined data flow”.

Graphical Data Flow Program (or Graphical Data Flow Diagram)—A GraphicalProgram which is also a Data Flow Program. A Graphical Data Flow Programcomprises a plurality of interconnected nodes (blocks), wherein at leasta subset of the connections among the nodes visually indicate that dataproduced by one node is used by another node. A LabVIEW VI is oneexample of a graphical data flow program. A Simulink block diagram isanother example of a graphical data flow program.

Graphical User Interface—this term is intended to have the full breadthof its ordinary meaning. The term “Graphical User Interface” is oftenabbreviated to “GUI”. A GUI may comprise only one or more input GUIelements, only one or more output GUI elements, or both input and outputGUI elements.

The following provides examples of various aspects of GUIs. Thefollowing examples and discussion are not intended to limit the ordinarymeaning of GUI, but rather provide examples of what the term “graphicaluser interface” encompasses:

A GUI may comprise a single window having one or more GUI Elements, ormay comprise a plurality of individual GUI Elements (or individualwindows each having one or more GUI Elements), wherein the individualGUI Elements or windows may optionally be tiled together.

A GUI may be associated with a graphical program. In this instance,various mechanisms may be used to connect GUI Elements in the GUI withnodes in the graphical program. For example, when Input Controls andOutput Indicators are created in the GUI, corresponding nodes (e.g.,terminals) may be automatically created in the graphical program orblock diagram. Alternatively, the user can place terminal nodes in theblock diagram which may cause the display of corresponding GUI Elementsfront panel objects in the GUI, either at edit time or later at runtime. As another example, the GUI may comprise GUI Elements embedded inthe block diagram portion of the graphical program.

Front Panel—A Graphical User Interface that includes input controls andoutput indicators, and which enables a user to interactively control ormanipulate the input being provided to a program, and view output of theprogram, while the program is executing.

A front panel is a type of GUI. A front panel may be associated with agraphical program as described above.

In an instrumentation application, the front panel can be analogized tothe front panel of an instrument. In an industrial automationapplication the front panel can be analogized to the MMI (Man MachineInterface) of a device. The user may adjust the controls on the frontpanel to affect the input and view the output on the respectiveindicators.

Graphical User Interface Element—an element of a graphical userinterface, such as for providing input or displaying output. Exemplarygraphical user interface elements comprise input controls and outputindicators.

Input Control—a graphical user interface element for providing userinput to a program. An input control displays the value input by theuser and is capable of being manipulated at the discretion of the user.Exemplary input controls comprise dials, knobs, sliders, input textboxes, etc.

Output Indicator—a graphical user interface element for displayingoutput from a program. Exemplary output indicators include charts,graphs, gauges, output text boxes, numeric displays, etc. An outputindicator is sometimes referred to as an “output control”.

Computer System—any of various types of computing or processing systems,including a personal computer system (PC), mainframe computer system,workstation, network appliance, Internet appliance, personal digitalassistant (PDA), television system, grid computing system, or otherdevice or combinations of devices. In general, the term “computersystem” can be broadly defined to encompass any device (or combinationof devices) having at least one processor that executes instructionsfrom a memory medium.

Measurement Device—includes instruments, data acquisition devices, smartsensors, and any of various types of devices that are configured toacquire and/or store data. A measurement device may also optionally befurther configured to analyze or process the acquired or stored data.Examples of a measurement device include an instrument, such as atraditional stand-alone “box” instrument, a computer-based instrument(instrument on a card) or external instrument, a data acquisition card,a device external to a computer that operates similarly to a dataacquisition card, a smart sensor, one or more DAQ or measurement cardsor modules in a chassis, an image acquisition device, such as an imageacquisition (or machine vision) card (also called a video capture board)or smart camera, a motion control device, a robot having machine vision,and other similar types of devices. Exemplary “stand-alone” instrumentsinclude oscilloscopes, multimeters, signal analyzers, arbitrary waveformgenerators, spectroscopes, and similar measurement, test, or automationinstruments.

A measurement device may be further configured to perform controlfunctions, e.g., in response to analysis of the acquired or stored data.For example, the measurement device may send a control signal to anexternal system, such as a motion control system or to a sensor, inresponse to particular data. A measurement device may also be configuredto perform automation functions, i.e., may receive and analyze data, andissue automation control signals in response.

Functional Unit (or Processing Element)—refers to various elements orcombinations of elements. Processing elements include, for example,circuits such as an ASIC (Application Specific Integrated Circuit),portions or circuits of individual processor cores, entire processorcores, individual processors, programmable hardware devices such as afield programmable gate array (FPGA), and/or larger portions of systemsthat include multiple processors, as well as any combinations thereof.

Automatically—refers to an action or operation performed by a computersystem (e.g., software executed by the computer system) or device (e.g.,circuitry, programmable hardware elements, ASICs, etc.), without userinput directly specifying or performing the action or operation. Thusthe term “automatically” is in contrast to an operation being manuallyperformed or specified by the user, where the user provides input todirectly perform the operation. An automatic procedure may be initiatedby input provided by the user, but the subsequent actions that areperformed “automatically” are not specified by the user, i.e., are notperformed “manually”, where the user specifies each action to perform.For example, a user filling out an electronic form by selecting eachfield and providing input specifying information (e.g., by typinginformation, selecting check boxes, radio selections, etc.) is fillingout the form manually, even though the computer system must update theform in response to the user actions. The form may be automaticallyfilled out by the computer system where the computer system (e.g.,software executing on the computer system) analyzes the fields of theform and fills in the form without any user input specifying the answersto the fields. As indicated above, the user may invoke the automaticfilling of the form, but is not involved in the actual filling of theform (e.g., the user is not manually specifying answers to fields butrather they are being automatically completed). The presentspecification provides various examples of operations beingautomatically performed in response to actions the user has taken.

Concurrent—refers to parallel execution or performance, where tasks,processes, or programs are performed in an at least partiallyoverlapping manner. For example, concurrency may be implemented using“strong” or strict parallelism, where tasks are performed (at leastpartially) in parallel on respective computational elements, or using“weak parallelism”, where the tasks are performed in an interleavedmanner, e.g., by time multiplexing of execution threads.

FIG. 1A—Computer System

FIG. 1A illustrates an exemplary computer system 82 configured toexecute a program, e.g., a graphical program, configured to programand/or utilize an interlock memory, according to some embodiments. Notethat while some of the embodiments are described in terms of a graphicalprogram, the techniques disclosed are broadly applicable to text basedprograms, as well, and so the described graphical program aspects of thedescribed embodiments should be considered to be exemplary only, i.e.,the novel techniques described with respect to graphical programs alsoapply to text based programs.

As shown in FIG. 1A, the computer system 82 may include a display deviceconfigured to display a graphical program as the graphical program iscreated and/or executed. The display device may also be configured todisplay a graphical user interface or front panel of the graphicalprogram during execution of the graphical program. The graphical userinterface may comprise any type of graphical user interface, e.g.,depending on the computing platform.

The computer system 82 may include at least one memory medium on whichone or more computer programs or software components according to oneembodiment of the present disclosure may be stored. For example, thememory medium may store one or more graphical (or text based) programswhich are executable to perform the methods described herein.Additionally, the memory medium may store a graphical (or text based)programming development environment application used to create and/orexecute such graphical (or text based) programs. The memory medium mayalso store operating system software, as well as other software foroperation of the computer system. Various embodiments further includereceiving or storing instructions and/or data implemented in accordancewith the foregoing description upon a carrier medium.

FIG. 1B—Computer Network

FIG. 1B illustrates a system including a first computer system 82 thatis coupled to a second computer system 90. The computer system 82 may becoupled via a network 84 (or a computer bus) to the second computersystem 90. The computer systems 82 and 90 may each be any of varioustypes, as desired. The network 84 can also be any of various types,including a LAN (local area network), WAN (wide area network), theInternet, or an Intranet, among others. The computer systems 82 and 90may execute a graphical program in a distributed fashion. For example,computer 82 may execute a first portion of the block diagram of agraphical program and computer system 90 may execute a second portion ofthe block diagram of the graphical program. As another example, computer82 may display the graphical user interface of a graphical program andcomputer system 90 may execute the block diagram of the graphicalprogram.

In one embodiment, the graphical user interface of the graphical programmay be displayed on a display device of the computer system 82, and theblock diagram may execute on a device coupled to the computer system 82.The device may include a programmable hardware element and/or mayinclude a processor and memory medium which may execute a real timeoperating system. In one embodiment, the graphical program may bedownloaded and executed on the device. For example, an applicationdevelopment environment with which the graphical program is associatedmay provide support for downloading a graphical program for execution onthe device in a real time system.

Exemplary Systems

Embodiments of the present disclosure may be involved with performingtest and/or measurement functions; controlling and/or modelinginstrumentation or industrial automation hardware; modeling andsimulation functions, e.g., modeling or simulating a device or productbeing developed or tested, etc. Exemplary test applications where thegraphical program may be used include hardware-in-the-loop testing andrapid control prototyping, among others.

However, it is noted that embodiments of the present invention can beused for a plethora of applications and is not limited to the aboveapplications. In other words, applications discussed in the presentdescription are exemplary only, and embodiments of the present inventionmay be used in any of various types of systems. Thus, embodiments of thesystem and method of the present invention is configured to be used inany of various types of applications, including the control of othertypes of devices such as multimedia devices, video devices, audiodevices, telephony devices, Internet devices, etc., as well as generalpurpose software applications such as word processing, spreadsheets,network control, network monitoring, financial applications, games, etc.

FIG. 2A illustrates an exemplary instrumentation control system 100which may implement embodiments of the invention. The system 100comprises a host computer 82 which couples to one or more instruments.The host computer 82 may comprise a CPU, a display screen, memory, andone or more input devices such as a mouse or keyboard as shown. Thecomputer 82 may operate with the one or more instruments to analyze,measure or control a unit under test (UUT) or process 150, e.g., viaexecution of software 104.

The one or more instruments may include a GPIB instrument 112 andassociated GPIB interface card 122, a data acquisition board 114inserted into or otherwise coupled with chassis 124 with associatedsignal conditioning circuitry 126, a VXI instrument 116, a PXIinstrument 118, a video device or camera 132 and associated imageacquisition (or machine vision) card 134, a motion control device 136and associated motion control interface card 138, and/or one or morecomputer based instrument cards 142, among other types of devices. Thecomputer system may couple to and operate with one or more of theseinstruments. The instruments may be coupled to the unit under test (UUT)or process 150, or may be coupled to receive field signals, typicallygenerated by transducers. The system 100 may be used in a dataacquisition and control application, in a test and measurementapplication, an image processing or machine vision application, aprocess control application, a man-machine interface application, asimulation application, or a hardware-in-the-loop validationapplication, among others.

FIG. 2B illustrates an exemplary industrial automation system 200 whichmay implement embodiments of the invention. The industrial automationsystem 200 is similar to the instrumentation or test and measurementsystem 100 shown in FIG. 2A. Elements which are similar or identical toelements in FIG. 2A have the same reference numerals for convenience.The system 200 may comprise a computer 82 which couples to one or moredevices or instruments. The computer 82 may comprise a CPU, a displayscreen, memory, and one or more input devices such as a mouse orkeyboard as shown. The computer 82 may operate with the one or moredevices to perform an automation function with respect to a process ordevice 150, such as MMI (Man Machine Interface), SCADA (SupervisoryControl and Data Acquisition), portable or distributed data acquisition,process control, advanced analysis, or other control, among others,e.g., via execution of software 104.

The one or more devices may include a data acquisition board 114inserted into or otherwise coupled with chassis 124 with associatedsignal conditioning circuitry 126, a PXI instrument 118, a video device132 and associated image acquisition card 134, a motion control device136 and associated motion control interface card 138, a fieldbus device270 and associated fieldbus interface card 172, a PLC (ProgrammableLogic Controller) 176, a serial instrument 282 and associated serialinterface card 184, or a distributed data acquisition system, such asFieldpoint system 185, available from National Instruments Corporation,among other types of devices.

FIG. 3A is a high level block diagram of an exemplary system which mayexecute or utilize programs, e.g., graphical programs. FIG. 3Aillustrates a general high-level block diagram of a generic controland/or simulation system which comprises a controller 92 and a plant 94.The controller 92 represents a control system/algorithm the user may betrying to develop. The plant 94 represents the system the user may betrying to control. For example, if the user is designing an ECU for acar, the controller 92 is the ECU and the plant 94 is the car's engine(and possibly other components such as transmission, brakes, and so on.)As shown, a user may create a graphical program that specifies orimplements the functionality of one or both of the controller 92 and theplant 94. For example, a control engineer may use a modeling andsimulation tool to create a model (graphical program) of the plant 94and/or to create the algorithm (graphical program) for the controller92.

FIG. 3B illustrates an exemplary system which may perform control and/orsimulation functions. As shown, the controller 92 may be implemented bya computer system 82 or other device (e.g., including a processor andmemory medium and/or including a programmable hardware element) thatexecutes or implements a graphical program. In a similar manner, theplant 94 may be implemented by a computer system or other device 144(e.g., including a processor and memory medium and/or including aprogrammable hardware element) that executes or implements a graphicalprogram, or may be implemented in or as a real physical system, e.g., acar engine.

In one embodiment, one or more graphical programs may be created whichare used in performing rapid control prototyping. Rapid ControlPrototyping (RCP) generally refers to the process by which a userdevelops a control algorithm and quickly executes that algorithm on atarget controller connected to a real system. The user may develop thecontrol algorithm using a graphical program, and the graphical programmay execute on the controller 92, e.g., on a computer system or otherdevice. The computer system 82 may be a platform that supports real timeexecution, e.g., a device including a processor that executes a realtime operating system (RTOS), or a device including a programmablehardware element.

In one embodiment, one or more graphical programs may be created whichare used in performing Hardware in the Loop (HIL) simulation. Hardwarein the Loop (HIL) refers to the execution of the plant model 94 in realtime to test operation of a real controller 92. For example, once thecontroller 92 has been designed, it may be expensive and complicated toactually test the controller 92 thoroughly in a real plant, e.g., a realcar. Thus, the plant model (implemented by a graphical program) isexecuted in real time to make the real controller 92 “believe” oroperate as if it is connected to a real plant, e.g., a real engine.

In the embodiments of FIGS. 2A, 2B, and 3B above, one or more of thevarious devices may couple to each other over a network, such as theInternet. In one embodiment, the user operates to select a target devicefrom a plurality of possible target devices for programming orconfiguration using a program, e.g., a graphical program. Thus the usermay create a graphical program on a computer and use (execute) thegraphical program on that computer or deploy the graphical program to atarget device (for remote execution on the target device) that isremotely located from the computer and coupled to the computer through anetwork.

Graphical software programs which perform data acquisition, analysisand/or presentation, e.g., for measurement, instrumentation control,industrial automation, modeling, or simulation, such as in theapplications shown in FIGS. 2A and 2B, may be referred to as virtualinstruments.

FIG. 4—Computer System Block Diagram

FIG. 4 is a block diagram 12 representing one embodiment of the computersystem 82 and/or 90 illustrated in FIGS. 1A and 1B, or computer system82 shown in FIG. 2A or 2B. It is noted that any type of computer systemconfiguration or architecture can be used as desired, and FIG. 4illustrates a representative PC embodiment. It is also noted that thecomputer system may be a general purpose computer system, a computerimplemented on a card installed in a chassis, or other types ofembodiments. Elements of a computer not necessary to understand thepresent description have been omitted for simplicity.

The computer may include at least one central processing unit or CPU(processor) 160 which is coupled to a processor or host bus 162. The CPU160 may be any of various types, including an x86 processor, e.g., aPentium class, a PowerPC processor, a CPU from the SPARC family of RISCprocessors, as well as others. A memory medium, typically comprising RAMand referred to as main memory, 166 is coupled to the host bus 162 bymeans of memory controller 164. The main memory 166 may store the one ormore programs configured to program and/or utilize an interlock memory.The main memory may also store operating system software, as well asother software for operation of the computer system.

The host bus 162 may be coupled to an expansion or input/output bus 170by means of a bus controller 168 or bus bridge logic. The expansion bus170 may be the PCI (Peripheral Component Interconnect) expansion bus,although other bus types can be used. The expansion bus 170 includesslots for various devices such as described above. The computer 82further comprises a video display subsystem 180 and hard drive 182coupled to the expansion bus 170. The computer 82 may also comprise aGPIB card 122 coupled to a GPIB bus 112, and/or an MXI device 186coupled to a VXI chassis 116.

As shown, a device 190 may also be connected to the computer. The device190 may include a processor and memory which may execute a real timeoperating system. The device 190 may also or instead comprise aprogrammable hardware element. The computer system may be configured todeploy a program, e.g., a graphical program to the device 190 forexecution of the graphical program on the device 190. The deployedgraphical program may take the form of graphical program instructions ordata structures that directly represents the graphical program.Alternatively, the deployed graphical program may take the form of textcode (e.g., C code) generated from the graphical program. As anotherexample, the deployed graphical program may take the form of compiledcode generated from either the graphical program or from text code thatin turn was generated from the graphical program. As noted above, insome embodiments, the program(s) may be text based.

Exemplary LDPC Decoder

Various embodiments discussed below with reference to FIGS. 5-16 involvetechniques associated with decoding error correcting code, andlow-density parity-check (LDPC) codes in particular. However, similartechniques may be applied to various algorithms in addition to and/or inplace of such decoding. LDPC codes are discussed for explanatorypurposes but are not intended to limit the scope of the presenttechniques. Embodiments of the interlock memory, performance modeling,hardware programming, memory sequence rearrangement, and/orself-addressing techniques discussed herein may be applied in variouscontexts for any of various algorithms which may or may not beexplicitly listed herein.

Examples of such algorithms include, without limitation: error controlcode, graph encoding/decoding, source coding, cryptography, maximumlikelihood detector, maximum a posteriori detector, compression,multiple-input multiple-output (MIMO) communications, beam-forming,beam-steering, differential equation solving, linear equation solving,linear algebra, optimization, detection and estimation, networking,machine learning, channel estimation, image processing, motion control,process control, bioinformatics, dynamic programming, big dataapplications, computational informatics, internet of things, etc. Thussequences of memory accesses may be based on, and/or derived from, oneor more of, without limitation: a parity-check matrix, a generatormatrix, a channel matrix, the number of nodes and/or edges in a decodingalgorithm, number of antennae, number of channel taps, compressionratio, angle range, degree of polynomial, number of equations, localand/or global maxima and/or minima, number of network nodes, etc.

Exemplary embodiments of linear algebra algorithms include, withoutlimitation: symmetric rank-k update, symmetric rank-2k update, Choleskyfactorization (decomposition of a positive-definite matrix into aproduct of a lower triangular matrix and its conjugate transpose),update Cholesky factorization (e.g., when a previously-decomposed matrixchanges in some way), lower upper (LU) factorization (decomposition of amatrix into a product of a lower triangular matrix and an uppertriangular matrix), QR factorization (decomposition of a matrix into aproduct of an orthogonal matrix Q and an upper triangular matrix R),update QR factorization, LQ factorization, triangular matrix inversion,reduction to Hessenberg form, reduction to bi-diagonal form, reductionto tri-diagonal form, triangular matrix Lyapunov equation solutions,triangular matrix Sylvester equation solutions, etc.

FIG. 5 shows one embodiment of an exemplary system that includes an LDPCdecoder. In the illustrated embodiment, system 500 includes encoder 510,decoder 520, modulator 530, and demodulator 540.

Encoder 510, in the illustrated embodiment, is configured to receive amessage to be transmitted, encode the message, e.g., using an LDPCgenerator matrix, and send the encoded message to modulator 530.

Modulator 530, in the illustrated embodiment, is configured to transformand/or group bits of the encoded message into symbols for wirelesstransmission to an antenna coupled to demodulator 540, i.e., to modulatethe encoded message for transmission. The wireless transmission mayconform to any of various transmission standards. In other embodiments,the transmission may not be wireless.

Demodulator 540, in the illustrated embodiment, is configured todemodulate the received signals to generate the encoded message.However, because of noise in transmission, it may be difficult todetermine the value of elements (e.g., the polarity of bits) in theencoded message. LDPC decoding may allow accurate reconstruction of theoriginal message in many situations.

Decoder 520, in the illustrated embodiment, is configured to reconstructthe original message, e.g., based on a parity check matrix (which isoften denoted using the letter ‘H’ and may be referred to as an “Hmatrix”), i.e., to decode the encoded message, thereby recovering theoriginal message. In the illustrated embodiment, decoder 520 includesinterlock memory 550, which may be configured to resolve or preventaccess hazards corresponding to operations associated with the paritycheck matrix. Embodiments of interlock memory 550 are described infurther detail below. Decoder 520 may include a pipelined data path forperforming memory accesses based on the parity check matrix, and varioustechniques disclosed herein may reduce latency in the pipelined datapath. Decoder 520 may be configured to transmit the decoded message torecipient hardware, not shown.

FIG. 6A shows a diagram illustrating an exemplary embodiment of an Hmatrix 620. In the illustrated embodiment, H matrix 620 is a paritycheck matrix and the number in each position indicates a cyclic shiftamount. The blank positions in H matrix 620 indicate a “don't care,”where the shift amount is not indicated, e.g., because it is not used.

In some embodiments, to decode a received signal using LDPC, decoder 520is configured to iterate through the layers of H matrix 620 (12 layersin the illustrated embodiment, which correspond to the rows of thematrix) to decode a received signal. For example, decoder 520 may readthe relevant positions in a layer, determine a minimum or maximum value,and update values for the layer based on the determined minimum ormaximum before proceeding to the next layer. In the illustratedembodiment, H matrix 620 is a 12 by 24 matrix, but any of various matrixsizes may be used in other embodiments.

FIG. 6B shows a diagram illustrating an exemplary embodiment of a betamatrix 640. Beta matrix 640, in the illustrated embodiment, encodesmemory read/write addresses for iterative processing of layers L1through L12 of H matrix 620. The encoded addresses correspond tolocations in interlock memory 550 in some embodiments. In theillustrated embodiment, beta matrix 640 is formed based on H matrix 620,and indicates the block index of positions in H matrix 620 that arerelevant (e.g., are not “don't cares”). For example, the position atlayer L1 and block index 4 in H matrix 620 has a value of 50, so thatblock index (4) is included in layer L1 in beta matrix 640. In theillustrated embodiment, beta matrix 640 also includes some blank “don'tcare” positions, which may be included in order to maintain a regularmatrix.

In the illustrated embodiment, beta matrix 640 includes only a third thenumber of columns included in H matrix 620. This may increase LDPCdecoding performance significantly, e.g., by up to three times in someembodiments, relative to performing operations for each position in Hmatrix 620.

Decoder 520, in some embodiments, is configured to process beta matrix640 iteratively in layers. In this example, to process the first layer,decoder 520 is configured to read memory locations 0, 4, 6, 8, 10, 12,and 13, modify at least some of the retrieved data, and write modifieddata back to at least some of the same locations. In one embodiment,decoder 520 is configured to calculate a minimum value for a given layerand calculate new values to write back to the same locations based onthe minimum value, before proceeding to the next layer. Thus, operationsfor a given layer may depend on the values written for the previouslayer. It may be desirable to pipeline memory accesses to improveperformance. However, note for example that the first location in layer2 is the same as the first location in layer 1 (both location 0). Thus,the read for layer 2 for location 0 is dependent on the write for layer1. In some embodiments, interlock memory 550 is configured to resolvethis dependency by waiting to perform the read for layer 2 until thewrite for layer 1 has completed. This stalling of the read may ensurecorrectness. However, reducing such stalling may be desirable in orderto increase read/write performance in pipelined implementations.

FIG. 6C shows a diagram illustrating one embodiment of a rearranged betamatrix 660. In the illustrated embodiment, rearranged beta matrix isformed by rearranging the positions in each row of beta matrix 640. Thisrearrangement may reduce read/write hazards and thus improve memoryaccess performance. For example, consider the accesses to location 0shown in bold. The read to location 0 for layer two has been shiftedsuch that the write to location 0 for layer one may be completed beforethe read for layer two, thus avoiding the need to stall in pipelinedimplementations. Thus, in various embodiments, sequences of memoryaccesses may be rearranged to improve memory performance. Thisrearranging may be performed automatically by a compiler and/or byconfiguration circuitry for interlock memory 550, for example.Techniques for performance modeling and selecting a rearranged sequenceof memory accesses are discussed below with reference to FIG. 12.

Referring now to FIG. 7A, a diagram illustrating a timeline of baselinedecoding 710 according to one embodiment is shown. In FIGS. 7A-7B,shaded rectangles indicate writes while non-shaded rectangles indicatereads.

In the example of FIG. 7A, reads occur for all locations for layer 1 ofbeta matrix 660, which takes at least eight cycles (assuming that oneread is initiated per cycle). All locations for layer 1 are thenwritten, taking at least another eight cycles in the illustratedexample. Processing for layer two then begins with a similar sequence ofreads followed by a sequence of writes.

Referring now to FIG. 7B, a diagram illustrating a timeline of pipelineddecoding 720 according to one embodiment is shown. In some embodiments,decoder 520 is configured to pipeline performance of memory accesses. Inthe illustrated example, reads for layer 2 begin as soon as reads forlayer 1 are finished. This may improve decoding performancesubstantially, e.g., by up to two times, relative to the baselinedecoding 710. Note that hazards must still be resolved or prevented(e.g., by inserting bubbles into the pipeline), but that rearranging asequence of memory accesses (e.g., using rearranged beta matrix 660) maygreatly reduce hazards in some embodiments.

Various pipelined memory access approaches are discussed herein, but itshould be noted that in other embodiments the disclosed techniques maybe used in the context of non-pipelined memory accesses.

In some embodiments, memory accesses may be fully pipelined such thatone or more reads and writes are issued every cycle if available,barring stalls to resolve hazards (not shown in FIG. 7B). In someembodiments, a memory may be multi-ported, allowing multiple readsand/or writes to issue in a given cycle, which may further increaseperformance relative to baseline decoding 710.

Exemplary Interlock Memory

FIG. 8A shows a block diagram illustrating one embodiment of interlockmemory 550. In the illustrated embodiment, interlock memory 550 includesmemory 810 and arbiter 830. Arbiter 830, in the illustrated embodiment,includes circuitry configured to receive read and write requests andgrant the requests, resolving or preventing/avoiding any hazards inaccessing memory 810 in the process. Interlock memory 550 may beconfigured to avoid memory access hazards without receiving informationindicating the hazards along with memory access requests correspondingto the sequence (e.g., interlock memory 550 may be configured to avoidthe hazards based on internal configuration). Arbiter 830 may beconfigured with memory access constraints for the sequence of memoryaccess requests, exemplary embodiments of which are described below withreference to FIGS. 9 and 14-15. These memory access constrains may bereferred to as “interlock information.” In some embodiments, interlockinformation is populated based on a specified sequence of memoryaccesses to be performed using interlock memory 550.

As used herein, circuitry being “configured with” or “configured using”memory access constraints is intended to include configurations inwhich: 1) the circuitry stores information, e.g., such as a datastructure or table which contains information regarding the memoryaccess constraints 2) the circuitry includes dedicated circuitryconfigured to implement the memory access constraints (e.g., an ASIC),and/or 3) the circuitry includes programmable circuitry that isprogrammed to implement the memory access constraints. Hazard circuitryconfigured with memory access constraints may be grouped in one locationor may be distributed (e.g., among stages of a pipelined memory accessdata path), in some embodiments.

Memory 810, in the illustrated embodiment, is configured to storereceived write data and provide read data, as allowed by arbiter 830. Insome embodiments, memory 810 is multi-ported to allow initiation ofmultiples reads and/or multiple writes in a given cycle. For example,dual-ported embodiments of memory 810 may allow initiation of writes totwo different locations in the same cycle. In some embodiments, memory810 includes multiple banks. In some embodiments, each bank allowsinitiation of only one read and one write per cycle, but different banksmay be accessed in the same cycle, e.g., concurrently.

FIG. 8B shows a block diagram illustrating another embodiment ofinterlock memory 550. In the illustrated embodiment, interlock memory550 includes address generator 820. Other elements of FIG. 8B may beconfigured as described above with reference to similarly numberedelements of FIG. 8A. Address generator 820, in the illustratedembodiment, includes circuitry configured to generate addresses for readand write requests. For example, interlock memory 550 may be configuredfor a particular array or matrix of memory accesses and addressgenerator 820 may be configured to assign addresses to requests based onthe sequence in which they are received. This may allow interlock memory550 to be self-addressing, in some embodiments, such that programs cansimply provide memory access requests, without address information, andinterlock memory 550 is configured to generate appropriate addresses forcorresponding accesses. In some embodiments, the sequence and/oraddresses of memory accesses in a program are determined duringcompilation, which may include compile-time execution as discussed belowwith reference to FIG. 11. In some embodiments, interlock memory 550includes one or more programmable hardware elements such that it iseasily reconfigurable for different programs and/or algorithms.

In some embodiments, interlock memory 550 may include additionalelements (not shown) configured to perform data forwarding, caching,prefetching, etc. associated with memory systems. In some embodiments,interlock memory 550 includes look-ahead circuitry configured to performand/or adjust one or more of caching, prefetching, data forwarding,etc., based on upcoming memory access addresses to improve performance.In various embodiments, interlock memory 550 is included in a devicesuch as an LDPC decoder or any of various appropriate devices. Thevarious techniques discussed herein may be used to improve performanceof any of various devices that include one or more interlock memories.

FIG. 9 shows a replication of the first five levels of rearranged betamatrix 660 and a corresponding interlock table 900. In variousembodiments, interlock tables are used to specify memory accessconstraints for a sequence of memory accesses. The interlock tables maybe stored in an interlock memory system and/or various circuitry may beotherwise configured with the memory access constraints (e.g., ratherthan maintaining the particular information specified by a giveninterlock table). The illustrated interlock table(s) are provided tofacilitate explanation of particular memory access constraints. In someembodiments, interlock table 900 is generated for a given program ormatrix and interlock memory 550 is configured with the memory accessconstraints specified by interlock table 900. In the illustratedembodiment, blank entries in interlock table 900 indicate that acorresponding access may be performed at any time because there are nohazards for that access.

In some embodiments, interlock memory 550 includes a read counter and awrite counter, which are initialized to zero before executing a program.In these embodiments, when performing a given read operation, interlockmemory 550 may be configured to compare the current write pointer to anentry in interlock table 900 corresponding to the read pointer and allowthe read only if the write pointer is greater than the entry. Forexample, consider the entry in row 3, column 3 in rearranged beta matrix660, which indicates location 0. The corresponding entry in interlocktable 900 contains a 9. Thus, in this embodiment, a read cannot beperformed from location 0 for row 3, column 3 in rearranged beta matrix660 until the write counter is at least 10. Note that when the writecounter is 10, for this sequence of memory operations, location 0 willhave been written (based on row 2, column 2 in rearranged beta matrix660), and so a potential hazard has been avoided. Similar techniques maybe used for writes. For example, in some embodiments, interlock memory550 is configured to grant a write only if the current read pointer isgreater than an entry in the interlock table indicated by the writepointer.

In some embodiments, interlock memory 550 is not configured to performhazard checks for write operations because hazards are already resolvedby dataflow. For example, for rearranged beta matrix 660, write afterread hazards are not present because the sequence already ensures thatreads for a given layer and the previous layer will be completed beforea given write to interlock memory 550 is requested (in this embodiment,writes for a given level are not initiated until reads for the entirerow have completed in order to find a minimum value).

FIG. 10 shows a flow diagram illustrating one embodiment of a method1000 for resolving or preventing data hazards. The method shown in FIG.10 may be used in conjunction with any of the computer systems, devices,circuits, elements, or components disclosed herein, among others. Invarious embodiments, some of the method elements (also referred toherein as “blocks”) shown may be performed concurrently, in a differentorder than shown, or may be omitted. Additional method elements may alsobe performed as desired. Flow begins at block 1010.

At block 1010, arrays for a linearized beta matrix (addr[96]) andinterlock table (interlock[96]) are maintained. These arrays may begenerated by using raster scan ordering on beta matrix 660 and interlocktable 900. One or both of these arrays are stored in interlock memory550 in some embodiments. In other embodiments, hazard circuitry isotherwise configured with information from one or both of these arrays.In the illustrated embodiment, rearranged beta matrix 660 may be used asan address table to generate addresses for received memory accessrequests. Flow proceeds to block 1020.

At block 1020, read and write pointers are initialized to zero. Flowproceeds to blocks 1030 and 1060.

At block 1030, a read request is received. Flow proceeds to decisionblock 1040.

At decision block 1040, it is determined whether the write pointer isgreater than the value in the interlock table at an index of the readpointer (interlock[read pointer]). If the write pointer is greater, flowproceeds to block 1050. Otherwise, flow proceeds back to decision block1040 (e.g., interlock memory 550 is configured to wait to grant the readrequest until the write pointer is greater than the value atinterlock[read pointer].

At block 1050, the location indicated by a position in the beta matrixat an index of the read pointer (addr[read pointer]) is read. Thus, therequested read operation is performed. Flow returns to block 1030.

At block 1060, a write request is received. Flow proceeds to block 1070.

At block 1070, data for the write request is written to the locationindicated by a position in the beta matrix at an index of the writepointer (addr[write pointer]). Thus, the requested write operation isperformed. In this embodiment, the write pointer is incremented (whichmay allow one or more waiting read requests to proceed). Flow returns toblock 1060.

In other embodiments, writes may be checked in a similar manner asutilized for reads in block 1040. In these embodiments, interlock memory550 is configured to stall a given write until the read pointer isgreater than interlock[write pointer]. This block is omitted in theillustrated embodiment because of the nature of the writes in theexample beta matrix 660, as discussed above. In still other embodiments,this block may be included for writes but block 1040 may be omitted forreads, e.g., for programs in which dataflow ensures that reads will notcause hazards.

Generating Interlock Information

FIG. 11 shows a flow diagram illustrating one embodiment of a method1100 for determining sequences and/or addresses of memory accesses for aprogram. The method shown in FIG. 11 may be used in conjunction with anyof the computer systems, devices, circuits, elements, or componentsdisclosed herein, among others. In various embodiments, some of themethod elements (or blocks) shown may be performed concurrently, in adifferent order than shown, or may be omitted. Additional methodelements may also be performed as desired. Flow begins at block 1110.

At block 1110, an input program is received. The program may includememory access operations, e.g., memory reads and/or writes. In someembodiments, a compiler receives the program for compilation for asystem that includes interlock memory 550. Flow proceeds to block 1120.

At block 1120, during compilation of the input program, information isgenerated by executing portions of the input program. Exemplarytechniques for executing portions of an input program during compilationare described in U.S. patent application Ser. No. 13/592,995, filed Aug.23, 2012 and titled “Compile Time Execution,” which is incorporatedherein by reference in its entirety. In some embodiments, thesetechniques are used to generate information for programming interlockmemory 550. Flow proceeds to block 1130.

At block 1130, addresses, a sequence, and/or memory access constraintsare generated for memory accesses in the input program based on theinformation. For example, in some embodiments, a compiler is configuredto generate one or more address tables (e.g., a beta matrix) and memoryaccess constraints for one or more interlock tables based on theinformation. In these embodiments, the compiler may be configured topartially execute the input program in order to determine the order ofmemory accesses in the program, addresses of the memory accesses, and/orhazards between the memory accesses. In some embodiments, the addressinformation is used for self-addressing. In some embodiments, hazardcircuitry for interlock memory 550 is configured with determined memoryaccess constraints and used to avoid hazards when executing the inputprogram. In some embodiments, this configuring may be performedautomatically, e.g., when the input program is to be compiled andprogrammed onto particular hardware. In some embodiments, the compileris configured to rearrange the sequence of memory operations in order toimprove performance using interlock memory 550. In some embodiments, therearrangement may alter compilation of the program to use the rearrangedsequence. Flow ends at block 1130.

In other embodiments, interlock information may be generated using othertechniques, in place of and/or in addition to compile time execution.For example, in some embodiments, a program, (e.g., a graphical program)may allow a user to enter an array or matrix corresponding to memoryaccesses and may be configured to generate interlock and/or addresstable(s) based on the entered information. For example, in oneembodiment, a program is configured to generate a beta matrix and one ormore interlock tables based on an input H matrix. In these embodiments,interlock information may be generated before compiling or executing anassociated input program at all. In other embodiments any of variousprograms, arrays, matrices, or other information associated with sets ofmemory accesses may be input to a program for configuring an interlockmemory.

In various embodiments, information (e.g., interlock information and/oraddress table(s)) may be transferred to an interlock memory at any ofvarious appropriate times. For example, the information may betransferred at the beginning of execution of a program or while theprogram is running. In some embodiments, sequence and/or interlockinformation may be updated during execution to adapt to changingconditions or improve one or more performance measures. Exemplaryperformance measures include, without limitation: throughput, latency,power consumption, reliability, resource utilization, and bit-errorrate.

Performance Estimates

In some embodiments, a program (e.g., a graphical program) is configuredto run cycle accurate simulations for interlock memory 550 for an inputprogram or a sequence of memory access operations. In some embodiments,the cycle accurate simulations are achieved or performed based onknowledge of the sequence of memory operations and the pipelined natureof memory accesses. For example, the number of stalls and number ofcycles for each stall may be determined for a particular sequence ofaccesses. In some embodiments, performance estimates may be generatedbased on such simulations. In some embodiments, sequences of memoryaccesses may be reordered to improve performance based on performanceestimates.

FIG. 12 shows a flow diagram illustrating one embodiment of a method forgenerating a rearranged or reordered sequence of memory accessoperations based on performance modeling. The method shown in FIG. 12may be used in conjunction with any of the computer systems, devices,circuits, elements, or components disclosed herein, among others. Invarious embodiments, some of the method elements shown may be performedconcurrently, in a different order than shown, or may be omitted.Additional method elements may also be performed as desired. Flow beginsat block 1210.

At block 1210, a performance estimate is determined. The performanceestimate may indicate an estimated number of cycles needed to perform asequence of memory access operations using interlock memory 550. Flowproceeds to decision block 1220.

At decision block 1220, it is determined whether the performanceestimate is acceptable. In some embodiments, this determination is basedon whether a particular number of iterations of blocks 1210-1230 havebeen performed. In some embodiments, this determination is based on adesired performance estimate or some other threshold. In someembodiments, an optimal ordering may be acceptable, while in otherembodiments sub-optimal orderings may be acceptable, e.g., based on someheuristic. If the performance is acceptable, flow proceeds to block 1240and flow ends. Otherwise, flow proceeds to block 1230.

At block 1230, the sequence of memory access operations is rearranged orreordered. One example of such reordering is discussed above withreference to FIGS. 6A-6C. Flow proceeds to block 1210. In someembodiments, the rearranged sequences and/or performance estimates arestored, and one of the rearranged sequences is selected based on theperformance estimates. In some embodiments, the selected sequence isused to compile a program and/or generate one or more interlock tablesfor interlock memory 550.

FIG. 13 shows a flow diagram illustrating one embodiment of a method forconfiguring an interlock memory system. The method shown in FIG. 13 maybe used in conjunction with any of the computer systems, devices,circuits, elements, or components disclosed herein, among others. Invarious embodiments, some of the method elements shown may be performedconcurrently, in a different order than shown, or may be omitted.Additional method elements may also be performed as desired. Flow beginsat block 1310.

At block 1310, a sequence of memory accesses for a program isdetermined. In some embodiments, the sequence is determined based oncompilation of an input program, e.g., per embodiments of the method ofFIG. 11, discussed above. The sequence may be reordered, in someembodiments to improve performance of interlock memory 550. Flowproceeds to block 1320.

At block 1320, information specifying memory access constraints isgenerated, based on the sequence of memory accesses. In this embodiment,the information is usable to avoid memory access hazards for thesequence of memory accesses. In some embodiments, the informationincludes one or more interlock tables. Flow proceeds to block 1330.

At block 1330, circuitry is configured using the information. In thisembodiment, the circuitry is included in or coupled to a memory. In thisembodiment, the circuitry is operable, after the configuration, toperform memory access requests to the memory corresponding to thesequence of memory accesses while avoiding memory access hazards. Inthis embodiment, the circuitry is configured to avoid the memory accesshazards without receiving other information indicating the memory accesshazards (e.g., a program may send the sequence of memory access requestsfor pipelined execution without indicating memory access hazards andwithout waiting for a particular memory access to complete beforesending dependent accesses). In some embodiment, the configuration isperformed using one or more programmable hardware elements. In someembodiments, the configuration is performed by downloading values intoone or more storage elements (e.g., one or more random access memories(RAMs) and/or one or more read only memories (ROMs)) included in and/orcoupled to interlock memory 550. In some embodiments, circuitry is alsoconfigured with address information for self-addressing the memoryaccesses using any of various appropriate techniques. In someembodiments, method 1300 is performed automatically based on variousinputs to a graphical program. Flow ends at block 1330.

Additional Interlock Memory Embodiments

FIG. 14 shows a block diagram illustrating one embodiment of a system1400 configured to implement an interlock memory. FIGS. 15A-B and 16A-Billustrate exemplary interlock tables, execution timelines, and methodsassociated with system 1400 in some embodiments. System 1400, in theillustrated embodiment, includes memory 1410 and hazard circuitry 1420.Memory 1410, in some embodiments, is a shared memory.

Hazard circuitry 1420, in the illustrated embodiment, is configured toreceive read and write requests and determine when to grant them inorder to resolve hazards based on write interlock table 1470 and readinterlock table 1480. Hazard circuitry 1420, in some embodiments, isintegrated into the memory system that includes memory 1410. Hazardcircuitry 1420, in the illustrated embodiment, includes write interlocktable 1470, read interlock table 1480, and circuitry configured tomaintain values for write pointer 1430, read pointer 1440, write tokens1450, and read tokens 1460. In various embodiments, hazard circuitry maybe distributed among elements of a pipelined data path. In someembodiments the information specified by write interlock table 1470 andread interlock table 1480 may not be directly stored, but may instead beused to configure hazard circuitry to perform various functionalitydescribed below.

Write tokens 1450 and read tokens 1460, in the illustrated embodiment,indicate how many reads or writes can be respectively performed beforethe next hazard, assuming the tokens are not incremented. For example,if read tokens 1460 has a value of three, then three reads can beperformed before a potential hazard, assuming the value is notincremented as a result of writes in the interim.

Write pointer 1430 and read pointer 1440, in the illustrated embodiment,indicate the how many operations of each type have been performed in asequence of memory accesses. In some embodiments, write pointer 1430 andread pointer 1440 are initialized to zero before execution of a givenprogram and are incremented for each write and read respectively.

Write interlock table 1470 and read interlock table 1480, in theillustrated embodiment, indicate values to be added to read tokens 1460or write tokens 1450 upon completion of particular read or writeoperations. These tables may be generated using compile time execution,generated based on user input, hard coded for a particular array ormatrix, etc. Exemplary embodiments of values for these tables arediscussed below with reference to FIGS. 15A-B.

Hazard circuitry 1420, in the illustrated embodiment, is configured toprovide write address, write enable, read address, and read enablesignals to memory 1410. In some embodiments, hazard circuitry 1420 isself-addressing and does not receive the address information duringexecution of a sequence of memory operations. In other embodiments, theaddresses are included in the read and write requests.

FIGS. 15A-B show exemplary embodiments of interlock tables and pipelinedexecution for different exemplary sequences of memory access operationsusing system 1400. These Figures show that rearranging a sequence ofmemory accesses (e.g., rearranging beta matrix 1510 to produce betamatrix1550 in the illustrated embodiment) may increase pipelinedperformance memory accesses using an interlock memory.

Beta matrix 1510, in the illustrated embodiment, includes two layersthat each include three blocks. In the illustrated embodiment, thepositions in the second layer of each table are shown using shadedblocks. Beta matrix 1550, in the illustrated embodiment, is generated byreordering the first layer of beta matrix 1510.

The execution diagrams illustrate exemplary pipelined performance ofmemory accesses for each of beta matrices 1510 and 1550. In theillustrated embodiment, upon completion of memory accesses for layer 2,execution begins again for layer 1. FIGS. 15A-B show that rearrangingthe order of memory access operations can improve performance, as thereordering to generate beta matrix 1550 has improved throughput relativeto beta matrix 1510.

Write interlock tables 1520 and 1560 and read interlock tables 1530 and1570, in the illustrated embodiment, include entries indicating thenumber of tokens to add upon completion of a corresponding operation inone of the beta matrices. For example, upon completion of a write tolocation 0 for layer 1 of beta matrix 1510, two read tokens are added,allowing at least two reads to occur before a stall to avoid the nextpotential hazard. As another example, upon completion of a write tolocation 4 for layer 2 of beta matrix 1510, four read tokens are added,allowing at least for reads to occur before a stall to avoid the nexthazard. In the illustrated embodiment, each read operation adds a singlewrite token. Note that in the illustrated embodiments, writes do notoccur immediately after a token is added. In this embodiment, this isthe result of the nature of the LDPC decoding operations, in which aprogram is configured to read all positions in a layer before writingany of them. Thus, when the first write request is received, in thisexample, the write tokens value has already been incremented to threebased on the three reads.

FIGS. 16A-B show exemplary embodiments of methods 1600 and 1650 foroperation of an interlock memory system, such as, for example, theembodiment of system 1400. The methods shown in FIGS. 16A-B may be usedin conjunction with any of the computer systems, devices, circuits,elements, or components disclosed herein, among others. In variousembodiments, some of the method elements shown may be performedconcurrently, in a different order than shown, or may be omitted.Additional method elements may also be performed as desired. Flow beginsat block 1602 for FIG. 16A and block 1652 for FIG. 16B. In someembodiments, these methods are performed (at least partially) inparallel.

Referring to FIG. 16A, at block 1602, hazard circuitry initializes aread tokens value. In the embodiments of FIGS. 15A-B, read tokens areinitialized to a value of three while write tokens are initialized to avalue of zero. Flow proceeds to block 1604.

At block 1604, hazard circuitry 1420 waits for a read request. Flowproceeds to decision block 1606.

At decision block 1606, hazard circuitry 1420 determines whether readtokens is greater than zero. If it is, flow proceeds to block 1608.Otherwise, flow remains at decision block 1606 until read tokens isgreater than zero. As shown by the dashed arrow from block 1666 in FIG.16B, read tokens may become greater than zero in response to a writeoperation, e.g., because performance of the write operation removes ahazard.

At block 1608 hazard circuitry 1420 fetches a token value from readinterlock table 1470 based on the read pointer. In self-addressingembodiments, hazard circuitry 1420 is also configured to retrieve theread address from an address table. Note that in some embodiments, theinterlock table may be configured to store addresses as well as indicatememory constraints. In these embodiments addresses may be retrieved fromthe interlock table in addition to interlock information. Flow proceedsto block 1610.

At block 1610, hazard circuitry 1420 increments the read pointer. Flowproceeds to block 1612.

At block 1612, hazard circuitry 1420 decrements the read tokens. In thisembodiment, each read may be thought of as consuming a read token, suchthat reads are allowed to occur only if a read token is available. Flowproceeds to block 1614.

At block 1614, hazard circuitry 1420 performs the requested readoperation. Flow proceeds to block 1616.

At block 1616, hazard circuitry 1420 adds the retrieved token value fromblock 1608 to write tokens. As shown by the dashed line from block 1616to decision block 1656 of FIG. 16B, this incrementing may allow awaiting write to proceed because performance of the read in block 1614may remove a hazard. Flow returns to block 1604.

Referring to FIG. 16B, at block 1652, hazard circuitry 1420 initializesa write tokens value. Flow proceeds to block 1654.

At block 1654, hazard circuitry 1420 waits for a write request. Flowproceeds to decision block 1656.

At decision block 1656, hazard circuitry 1420 determines whether writetokens is greater than zero. If it is, flow proceeds to block 1658.Otherwise, flow remains at decision block 1656 until write tokens isgreater than zero.

At block 1658 hazard circuitry 1420 fetches a token value from writeinterlock table 1480 based on the write pointer. In self-addressingembodiments, hazard circuitry 1420 is also configured to retrieve thewrite address from an address table. Flow proceeds to block 1660.

At block 1660, hazard circuitry 1420 increments the write pointer. Flowproceeds to block 1662.

At block 1662, hazard circuitry 1420 decrements the write tokens. Inthis embodiment, each write may be thought of as consuming a writetoken, such that writes are allowed to occur only if a write token isavailable. Flow proceeds to block 1664.

At block 1664, hazard circuitry 1420 performs the requested writeoperation. Flow proceeds to block 1666.

At block 1666, hazard circuitry 1420 adds the retrieved token value fromblock 1608 to read tokens. As shown by the dashed line from block 1666to decision block 1606 of FIG. 16A, this incrementing may allow awaiting read to proceed because performance of the write in block 1654may remove a hazard. Flow returns to block 1654.

Note that FIGS. 14-16B illustrate exemplary token-based techniques forinterlock tables while FIG. 9 illustrates an exemplary counter-basedtechnique (e.g., each entry in interlock table 900 indicates the countor number of memory operations that must occur before a given access isallowed to proceed). In other embodiments, various other techniques maybe used to indicate hazards using an interlock table and stall executionof memory accesses based on the interlock table. The disclosedembodiments are provided herein for exemplary purposes and are notintended to limit interlock table implementations in other embodiments.

In some embodiments, an interlock memory system may be configured withsets of memory access constraints for multiple different programs. Inthese embodiments, an indication of one of the different programs may betransmitted to the interlock memory system and the corresponding memoryaccess constraints may be used to handle memory access requests for agiven program. In various programmable embodiments, circuitry configuredwith a first set of memory access constraints for a first program may bere-configured with a second, different set of memory access constraintsfor a second program.

Re-configurable circuitry may be advantageous in various applications.For example, a user equipment device (UE) that includes re-configurableinterlock circuitry may receive a new H matrix or beta matrix forLDPC-backed communications. In these embodiments, the UE may beconfigured to generate a program to implement LDPC decoding based on thereceived matrix. In other embodiments, the program may be transferreddirectly to the UE. In these embodiments, the UE may be configured tore-order memory accesses corresponding to the matrix in order to reducepipelined execution time. In these embodiments, the UE is configured toconfigure hazard circuitry with a new set of memory access constraintsfor the communication. In some embodiments, the UE is configured toreceive a set of memory access constraints for the program rather thangenerating them internally. In various embodiments, the UE may decodeincoming wireless transmissions using the program and an interlockmemory configured with the memory access constraints. In someembodiments, a UE that includes interlock memory configured to storememory access constraints for multiple programs may be configured to usedifferent parity check techniques for different communications, e.g.,when a device communicates via different networks.

Referring now to FIG. 17, one generalized embodiment of a method 1700for operating an interlock memory is shown. The method shown in FIG. 17may be used in conjunction with any of the computer systems, devices,circuits, elements, or components disclosed herein, among others. Invarious embodiments, some of the method elements shown may be performedconcurrently, in a different order than shown, or may be omitted.Additional method elements may also be performed as desired. Flow beginsat block 1710.

At block 1710, a sequence of memory access requests is received for amemory. In some embodiments, execution of a program generates thesequence of memory access requests. Flow proceeds to block 1720.

At block 1720, the memory access requests are granted by circuitryconfigured with memory access constraints for the memory accessrequests. In this embodiment, the circuitry is configured to grant thememory access requests subject to the memory access constraints, therebyavoiding memory access hazards for a sequence of memory accessescorresponding to the sequence of memory access requests. The circuitrymay include one or more storage elements configured to store informationspecifying the memory access constraints. The circuitry may beprogrammable to grant the memory access requests based on the memoryaccess constraints (e.g., the circuitry may include programmablehardware elements). The circuitry may be dedicated to granting thememory access requests based on the memory access constraints (e.g., thecircuitry may be an ASIC). Flow ends at block 1720.

Referring now to FIG. 18, one generalized embodiment of a method 1800for reordering a sequence of memory accesses is shown. The method shownin FIG. 18 may be used in conjunction with any of the computer systems,devices, circuits, elements, or components disclosed herein, amongothers. In various embodiments, some of the method elements shown may beperformed concurrently, in a different order than shown, or may beomitted. Additional method elements may also be performed as desired.Flow begins at block 1810.

At block 1810, a specified sequence of memory accesses is stored. Thespecified sequence may be received from another computing system orgenerated by the computing system storing the sequence, e.g., usingcompile-time execution. In this embodiment, the specified sequencecorresponds to a function to be performed (such as LDPC decoding, forexample) and has first memory access constraints. Flow proceeds to block1820.

At block 1820, the specified sequence is reordered to create a reorderedsequence of memory accesses that has second, different memory accessconstraints. In this embodiments, the reordered sequence of memoryaccesses is usable to access a memory to perform the function. Thereordering may improve performance of the memory accesses using aninterlock memory, as discussed above with reference to FIGS. 6C and 12.The reordering may be performed multiple times and a particularreordered sequence may be selected based on one or more criteria. Thereordered sequence of memory accesses may have a greater memory accessefficiency than the original sequence of memory accesses. For example,the reordered sequence may be usable to perform the function in asmaller number of clock cycles than a number of clock cycles used toperform the function using the original sequence. Flow ends at block1820.

In some embodiments, a computing system may compile a program toimplement the reordered sequence of memory accesses. Execution of theprogram may generate a sequence of memory access requests correspondingto the reordered sequence of memory accesses. The memory access requestsmay be performed using an interlock memory.

Referring now to FIG. 19, one generalized embodiment of a method 1900for operating a self-addressing memory is shown. The method shown inFIG. 19 may be used in conjunction with any of the computer systems,devices, circuits, elements, or components disclosed herein, amongothers. In various embodiments, some of the method elements shown may beperformed concurrently, in a different order than shown, or may beomitted. Additional method elements may also be performed as desired.Flow begins at block 1910.

At block 1910, memory access requests are received, corresponding to aspecified sequence of memory accesses, but the memory access requests donot include address information. For example, the memory access requestsmay be generated by an LDPC decoder program. The requests may specifywhether a read or write operation is desired, but do not specify alocation to be read/written. Flow proceeds to block 1920.

At block 1920, addresses are assigned to the memory accesses requestsfor the specified sequence of memory accesses. This may be performed byaddressing circuitry configured with address information for thespecified sequence of memory accesses. The addressing circuitry may beprogrammable or may be dedicated to a particular sequence. Flow proceedsto block 1930.

At block 1930, the sequence of memory accesses is performed using theassigned addresses. Flow ends at block 1930.

Referring now to FIG. 20, a sequence of memory accesses 2020 forCholesky factorization of a symmetric matrix 2010 is shown. In theillustrated embodiment, symmetric matrix 2010 is divided into 2×2submatrix blocks B0-B3. Sequence 2020 captures the Choleskeyfactorization algorithm's sequence of read/modify/writes for thesesubmatrix blocks. In the illustrated embodiment, for the second row, forexample, B2 and B0 are read, potentially modified, and written beforeproceeding to row three. In other embodiments, similar sequences may bedetermined for any of various matrix operations. In some embodiments,various techniques described herein may be utilized for such sequences.For example, interlock information may be generated for sequence 2020,sequence 2020 may be rearranged to reduce memory access hazards,addresses may be generated for sequence 2020, etc.

Although the embodiments above have been described in considerabledetail, numerous variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

We claim:
 1. An apparatus, comprising: a memory; and first circuitrycoupled to or comprised in the memory and configured to: receive asequence of memory access requests for the memory, wherein the sequenceof memory access requests is configured to access locations associatedwith entries in a matrix, wherein the first circuitry is configured withmemory access constraints for the sequence of memory access requests;and grant the sequence of memory access requests subject to the memoryaccess constraints, thereby avoiding memory access hazards for asequence of memory accesses corresponding to the sequence of memoryaccess requests.
 2. The apparatus of claim 1, wherein the apparatusincludes pipeline circuitry configured to pipeline performance of thesequence of memory accesses; and wherein the first circuitry isconfigured to stall pipelined performance of the sequence of memoryaccess requests based on the memory access constraints.
 3. The apparatusof claim 1, wherein the first circuitry is further configured to:maintain a read counter for read accesses to the memory and a writecounter for write accesses to the memory; and grant the memory accessrequests based on the read counter and the write counter.
 4. Theapparatus of claim 3, wherein the memory access constraints indicate aread pointer value that must be attained prior to performance of a writeoperation.
 5. The apparatus of claim 3, wherein the memory accessconstraints indicate a write pointer value that must be attained priorto performance of a read operation.
 6. The apparatus of claim 1, whereinthe memory access constraints specify, for a particular memory accessoperation, a number of subsequent read or write operations in thesequence of memory access requests to grant based on performance of theparticular memory access operation.
 7. The apparatus of claim 1, whereinthe sequence of memory accesses includes read, modify, and writeoperations for locations indicated by different rows of a sparse matrix,wherein read, modify, and write operations to a location for a first roware to be completed prior to a read operation to the location for asecond row.
 8. The apparatus of claim 1, wherein the first circuitrycomprises one or more tables configured to store information indicatingthe memory access constraints.
 9. The apparatus of claim 1, wherein thefirst circuitry includes circuitry distributed among pipeline stages ofa memory access pipeline, wherein the circuitry is programmed to enforcethe memory access constraints.
 10. The apparatus of claim 1, wherein thememory is a multi-ported memory configured to begin performance of aplurality of read requests and/or a plurality of write requests in thesame cycle.
 11. The apparatus of claim 1, wherein the first circuitry isconfigured with sets of memory access constraints for a plurality ofdifferent sequences of memory access requests and is configured toutilize one of the sets of memory access constraints based on anindication of one of the plurality of different sequences of memoryaccess requests.
 12. The apparatus of claim 1, receive configurationinformation for a second set of memory access constraints for a secondsequence of memory access requests; and grant the second sequence ofmemory access requests subject to the second set of memory accessconstraints, thereby avoiding memory access hazards for a secondsequence of memory accesses corresponding to the second sequence ofmemory access requests.
 13. A method, comprising: receiving, by firstcircuitry coupled to or comprised in a memory, a sequence of memoryaccess requests for the memory, wherein the first circuitry isconfigured with memory access constraints for the sequence of memoryaccess requests; and granting, by the first circuitry, the memory accessrequests subject to the memory access constraints, thereby avoidingmemory access hazards for a sequence of memory accesses corresponding tothe sequence of memory access requests.
 14. The method of claim 13,wherein the granting the memory access requests subject to the memoryaccess constraints includes stalling pipelined performance of thesequence of memory accesses based on the memory access constraints. 15.The method of claim 13, further comprising: maintaining a read counterfor read accesses to the memory and a write counter for write accessesto the memory; wherein the granting the memory access requests subjectto the memory access constraints is based on the read counter and thewrite counter.
 16. The method of claim 15, wherein the memory accessconstraints indicate a read pointer value that must be attained for thesequence of memory accesses prior to performance of a particular writeoperation.
 17. The method of claim 15, wherein the memory accessconstraints indicate a write pointer value that must be attained for thesequence of memory accesses prior to performance of a particular readoperation.
 18. The method of claim 13, wherein the memory accessconstraints specify, for a particular memory access operation, a numberof subsequent read or write operations in the sequence of memory accessrequests to allow without stalling based on performance of theparticular memory access operation.
 19. The method of claim 13, whereinthe sequence of memory accesses includes read/modify/write operationsfor different rows of a matrix, wherein a read/modify/write to alocation for a first row is to be completed prior to a read operation tothe location for a second row.
 20. The method of claim 13, furthercomprising storing information specifying the memory access constraintsin one or more tables.
 21. The method of claim 13, further comprisingimplementing the memory access constraints using programmable circuitry.22. The method of claim 13, further comprising: performing memoryaccesses for the sequence of memory access requests, wherein theperforming includes performing, by a multi-ported memory, multiple reador write memory accesses in the same cycle.
 23. The method of claim 13,wherein the first circuitry is configured with sets of memory accessconstraints for a plurality of different sequences of memory accessrequests, the method further comprising: selecting one of the sets ofmemory access constraints based on an indication of one of the pluralityof different sequences of memory access requests.
 24. The method ofclaim 13, further comprising: receiving configuration information for asecond set of memory access constraints for a second sequence of memoryaccess requests; and granting the second sequence of memory accessrequests subject to the second set of memory access constraints, therebyavoiding memory access hazards for a second sequence of memory accessescorresponding to the second sequence of memory access requests.
 25. Alow-density parity-check (LDPC) decoder, comprising: a memory configuredto store data for a received transaction; first circuitry configured toperform sequentially, for rows of an LDPC parity check matrix: retrieveportions of the stored data from the memory, wherein the portions of thestored data are indicated by entries in the rows of the LDPC paritycheck matrix; process the retrieved portions; and modify the stored databased on the processing; and second circuitry configured with memoryaccess constraints for the sequential retrievals and modifications,wherein the second circuitry is configured to grant the retrievals andmodifications subject to the memory access constraints, thereby avoidingmemory access hazards among the retrievals and modifications.
 26. TheLDPC decoder of claim 25, wherein the LDPC parity check matrix and thememory access constraints are programmable.
 27. The LDPC decoder ofclaim 25, wherein the memory access constraints indicate a number ofread operations that must occur prior to performance of a writeoperation.
 28. The LDPC decoder of claim 25, wherein the memory accessconstraints indicate a number of write operations that must occur priorto performance of a read operation.
 29. The LDPC decoder of claim 25,wherein the memory access constraints specify, for a particular memoryaccess operation, a token value indicating a number of subsequent reador write operations to allow based on performance of the particularmemory access operation.
 30. The LDPC decoder of claim 25, wherein theLDPC parity check matrix is a beta matrix generated based on an Hmatrix.